For example, content data of a movie, music, or the like is recorded on a DVD. Such content data is digital data. Thus, even if the content data is copied, the data itself does not deteriorate. As a result, if one content data which is to be a source is obtained, it is possible to make many copies of content data with identical content. In addition, with the copied content data as a source, it is possible to further make copies of content data with identical content.
Thus, the information recording medium, such as a DVD, with such digital data recorded thereon is equipped with a measure to restrict the copy for copyright protection or the like. The most general of the measures is to encrypt the content data.